As can be imagined, scanning instruments are an important aspect of space science. In addition to their use in scanning the earth and other planets, x-ray, gamma-ray, and similar scanning instruments perform additional functions important in space exploration. Examples of such instruments are sensors, telescopes and electronic devices carried by space platforms such as a space shuttle, a space station, by experimental balloons, and by free-flying spacecraft.
To achieve the scan pattern a drive means must be provided which impart to the payload an oscillatory motion. Such drive means are generally known, particularly in space exploration. Various forms of machines or apparatus have been employed for controllably conferring on scanning instruments predetermined scan patterns. They include control moment gyroscopes, reaction wheels, torque motors, reaction control systems, and various combinations of such apparatus.
One disadvantage of utilizing such scan generating devices is power consumption. For this reason, whether scanning with a ground-based, space-based, or balloon-borne gimballed payload a preferred drive means which is particularly effective is a rotating unbalanced-mass, or RUM, device. This device is the subject of our U.S. Pat. No. 5,129,600.
RUM devices are a new and efficient way to generate scan patterns using gimballed payloads such as x-ray telescopes or other scientific instruments. A RUM device consists of a mass, m, on a lever arm r, located at a distance, d, from the center-of-mass of the gimballed payload on which it is mounted. The mass is driven at a constant angular velocity .omega. which produces a cyclical centrifugal force m.omega..sup.2 r on the payload. This force, in turn, produces a cyclic torque, about the payload center-of-mass, with an amplitude of m.omega..sup.2 rd. The period of this cyclic torque is the same as the period of rotation of the RUMs. Two RUM devices are required to scan with-gimballed payloads. RUMs are mounted on each end of the payload and they rotate 180.degree. out-of-phase producing a cyclic torque couple having an amplitude of 2m.omega..sup.2 rd.
RUM devices are superior to previous scanning devices in terms of power, weight, cost, and accuracy, but such apparatus is still not totally satisfactory. Even though operating power requirements are less than those required for operating other scan generating devices, the required positioning and repositioning, or pointing, means are subject to improvement. RUM devices currently require an auxiliary control system to position and reposition the scan pattern relative to a target or a number of targets. Such control means confer on the payload or instrument a slow complementary motion that keeps the Scan centered on the target.
Prior methods of generating control torques for pointing freeflying/tethered satellites and gimballed payloads employed reaction wheels, control moment gyroscopes (CMGs), reaction control system (RCS) thrusters, and gimbal torque motors.
Pointing and scanning with reaction wheels, CMGs, and gimbal torque motors characteristically require a great deal of power. Reaction wheels and CMGs also require a momentum desaturation system. RCS systems can only generate control torques until the RCS propel]ant is depleted. In addition, they are also normally nonlinear devices that produce either full thrust or no thrust. This characteristic makes them unsuitable for applications where precise pointing and scanning are required. Gimbal torque motors require gimbals and a base structure to torque against, which renders them unsuitable for free-flying spacecraft and satellites. Also, reaction forces and torques acting against the base structure tend to couple back into the payload or instrument being pointed, causing pointing/scanning errors and/or stability problems. It can be seen, then, that despite the desirability of RUMs, there is room for improvement in their operation. A needed improvement is the elimination of such pointing, that is, positioning and repositioning, equipment, normally referred to as auxiliary control systems. We have now found that this can be accomplished.